As a built-in antenna of a small and thin radio communication terminal such as a mobile telephone, a plane antenna with a short-circuit plate having a structure as shown in FIG. 18 is well known.
In FIG. 18, in a plane antenna 210 with a short-circuit plate, a radiator conductor plate 212 which is a radiator conductor is arranged on a grounded conductor plate, that is, a ground plate 211, and the radiator conductor plate 212 is connected to the ground plate 211 via a short-circuit plate 213. Power is supplied to a feeding point 212a on the radiator conductor plate 212 by a feeder cable 214 from a power-feeding source 215 through a hole 211a bored in the ground plate 211.
The plane antenna 210 with a short-circuit plate shown in FIG. 18 is known to resonate at a frequency when the length of L0 shown in the drawing is about .lambda.g/4 (.lambda.g indicates an effective wavelength).
Meanwhile, in such a plane antenna, for example, to apply this antenna to a system having 2 or more built-in radio terminals, a multifrequency antenna for receiving two or more different frequency bands together may be required.
Conventionally, as a multifrequency antenna for receiving two or more different frequency bands, the constitution shown in FIG. 19 or 20 is known.
A multifrequency antenna 220 shown in FIG. 19 is structured so that two radiator conductor plates 222-1 and 222-2 different in size are arranged in parallel with a ground plate 221, and these two radiator conductor plates 222-1 and 222-2 are connected to the ground plate 221 via short-circuit plates 223-1 and 223-2 respectively, and power is supplied to a feeding point 222-1a on the radiator conductor plate 222-1 from a power-feeding source 225-1 via a feeder cable 224-1, and power is supplied to a feeding point 222-2a on the radiator conductor plate 222-2 from a power-feeding source 225-2 via a feeder cable 224-2.
Namely, the multifrequency antenna 220 shown in FIG. 19 is structured so that two single-frequency plane antennas resonating in different frequency bands respectively are arranged side by side and by use of such a constitution, a problem arises that the arrangement of the two single-frequency plane antennas increases the mounting area. A multifrequency antenna 230 shown in FIG. 20 is structured so that two radiator conductor plates 232-1 and 232-2 different in size are stacked and arranged on a ground plate 231, and these two radiator conductor plates 232-1 and 232-2 are connected to the ground plate 231 via short-circuit plates 233-1 and 233-2 respectively, and power is supplied to a feeding point 232-1a on the radiator conductor plate 232-1 from a power-feeding source 235-1 via a feeder cable 234-1, and power is supplied to a feeding point 232-2a on the radiator conductor plate 232-2 from a power-feeding source 235-2 via a feeder cable 234-2.
Namely, the multifrequency antenna 230 shown in FIG. 20 is structured so that two single-frequency plane antennas resonating in different frequency bands respectively are stacked and arranged and by use of such a constitution, a problem arises that the stacking arrangement of the two single-frequency plane antennas increases the height of the mounting portion and increases the mounting volume.
As mentioned above, in a conventional multifrequency antenna, compared with a single-frequency plane antenna with a short-circuit plate, the mounting area and mounting volume are larger and it may cause obstacles to miniaturization and thinning of a radio terminal accommodating this multifrequency antenna.